1. Field of the Invention
The present invention relates to an electric leak detecting apparatus and method for detecting an electric leak caused by deteriorated insulation between vehicle body and battery pack in an electric motorcar provided with a high voltage battery pack insulated from the vehicle body electrically to prevent electrical shocks.
2. Description of the Related Art
An electric motorcar, which uses an electric power as its driving source, is provided with a battery pack composed as a closed circuit and separated from the vehicle body. The battery pack functions as a ground to prevent electric shocks to be caused by a high voltage set battery used as a driving power source. If the insulating characteristics of the battery pack are deteriorated due to a change of the material quality thereof and/or deposit thereon, however, the high voltage battery pack is connected electrically to the vehicle body. And, this may cause a person to have an electric shock when he/she touches the vehicle body. Because, a leak current from the battery pack flows through him/her body.
An electric leak is a sum of leak currents flowing in all insulated portions. It is thus difficult to identify a system in which the current flows through. In order to detect such an electric leak, there have been proposed two methods so far; an AC method, which detects an electric leak resistance by applying an AC signal to an object vehicle body insulated from direct currents by a condenser and a transformer, and a non-insulating DC method, which measures an electric leak resistance as a direct current without using any of the condenser and the transformer. For example, the official report of Unexamined Published Japanese Patent Application No.57-119263 has disclosed an embodiment that adopts the AC method that uses a transformer and the official report of Unexamined Published Japanese Patent Application No.60-262069 has disclosed an embodiment that adopts the DC method.
FIG. 20 shows an equivalent circuit from which an electric leak is detected. Usually, an electric leak should be modeled with a distributed constant concept. If it is taken into consideration that a current leaks from the potential node of every set battery, however, the equivalent circuit of the battery pack 3 is represented by the set battery 3a and an electric leak admittance 3b noted by a concentrated constant using a resistor and a condenser. In FIG. 20, I0 to In are leak currents, each flowing from a node to the vehicle body. The sum I of the leak currents is thus equal to the sum of the currents I0 to In because of the superposition theory of currents.
[Equation 1] ##EQU1##
If the admittance in a battery is so low that it may be ignored, the admittance between the vehicle ground and the set battery 3a can be represented as the sum of admittance elements.
[Equation 2] ##EQU2##
And, the battery pack 3 shown in FIG. 20 can further be simplified as an AC equivalent circuit as shown in FIG. 21.
The AC method, when used for detecting electric leaks, is characterized as follows:
(1) Since the object is insulated, no DC current flows in the object. The method is thus excellent in the aspect of safety. PA1 (2) An admittance to be detected includes a floating capacitance of the object vehicle. The admittance thus includes an error. PA1 (3) Although a lower frequency AC signal is advantageous to improve the electric leak detecting accuracy. However, the use of such a lower frequency deteriorates the responsiveness. In addition, the input condenser must be expanded more in size. PA1 (4) Detected values are apt to be affected and varied by the floating capacitance of the object vehicle. In addition, the floating capacitance is also changed according to the external shape of the vehicle, so it is difficult to decide a criterion for electric leaks.
For example, if a conventional configuration as shown in FIG. 22 is taken for the AC method, an electric leak resistance is found as an absolute value of admittance. As shown in the vector in FIG. 23, therefore, an admittance absolute value .vertline.YLEAK.vertline. is used in place of an electric leak resistance for convenience sake. Refer to Equation 3.
[Equation 3] ##EQU3##
In the above case, however, the electric leak judgment will include an error due to the floating capacitance Cs. In order to reduce the influence of the floating capacitance Cs thereby to improve the measuring accuracy, the phase angle .o slashed. must be reduced. And, the detecting signal frequency must be lowered to reduce the phase angle .o slashed..
On the other hand, the DC method can solve the problems caused by the AC method described in (1) to (4). The AC method, however, cannot solve a problem that the measuring accuracy is still low due to an error of the electric leak resistance of the battery pack itself. Further there is such problem that the detection side and driving side are connected with direct current on account of the detection.